1. Field of the Invention
The present invention relates to an X-ray image intensifier and a method of manufacturing the same, and more particularly to an improvement of the output phosphor film formed in the output screen of the image intensifier.
2. Description of the Related Art
As is shown in FIG. 1, an X-ray image intensifier generally comprises vacuum envelope 1, input screen 2 located near the input end of envelope 1, and output screen 3 located near the output end of envelope 1. Anode 4 and focusing electrode 5, both being hollow cylinders, are provided within envelope 1. Anode is located near output screen 3, and focusing electrode 5 extends between places 2 and 3, with its circumferential surface extending parallel to the inner surface of envelope 1.
Output plate 3 has such a structure as is shown in FIG. 2. Output phosphor film 12 is formed on face plate 11, and aluminum layer 13 is formed on output phosphor film 12. Output screen 3 is positioned in vacuum envelope 1 such that face plate 11 facing outward.
Output screen 3 is conventionally made by the following steps:
(1) (Zn, Cd) S:Ag phosphor having a particle size of 1 to 3 .mu.m is coated on face plate 11 by a slurry coating technique, etc., thus forming phosphor film 12 having a thickness of 5 to 10 .mu.m. PA0 (2) A nitrocellulose coating is formed on phosphor film 12. PA0 (3) A first aluminum layer having a thickness of about 1000 .ANG. is formed on the nitrocellulose coating, by means of vapor-deposition method or the like. PA0 (4) Face plate 11 is heated, thereby decomposing the nitrocellulose coating. The decomposed nitrocellulose evaporates through pores of the first aluminum layer. PA0 (5) A second aluminum layer having a thickness of 2000 to 3000 .ANG. is formed on the first aluminum layer. The first and second aluminum layer collectively constitutes aluminum layer 13. PA0 (1) To reflect, among the rays emitted from the phosphor in various directions, those rays which are emitted from the phosphor toward an electron beam source, thereby enhancing luminance. PA0 (2) To prevent output phosphor film 12 from being charged electrically. PA0 (3) To protect output phosphor film 12 from the alkali atmosphere within envelope 1, e.g., sodium atmosphere.
Layer 13, which is a component of output screen 3, performs the following functions:
To perform all these functions, aluminum layer 13 must be about 4000 to 5000 .ANG. thick. Layer 13 is formed by the separate of two steps, as has been mentioned above, for the following reason. The phosphor forming film 12 has a particle size of about 1 to 3 .mu.m. Due to this relatively large particle size, gaps among the phosphor particles are formed which are greater than the deposit aluminum particles forming layer 13. Hence, when aluminum layer 13 vapor-deposited directly on output phosphor film 12, the aluminum particles are mixed into film 12. To prevent them from mixing into film 12, nitrocellulose is coated on output phosphor film 12. Here arises a problem. When aluminum is deposited at once on the nitrocellulose coating, thus forming layer 13 to a thickness of about 4000 to 5000 .ANG., the layer 13 has pores too small to allow the passage of the thermally decomposed nitrocellulose. Thus, two aluminum layers are formed, one after the other, to form layer 13.
In short, the conventional method of making the output screen 3 of an X-ray image intensifier has a drawback. It must involue a step of forming a nitrocellulose coating, a step of removing the coating, and various steps which must be performed to help successfully form and remove this coating. These steps renders the method complex.
In general, characteristics of an output screen for use in an X-ray image intensifier can be expressed by cathode-luminescence brightness, resolution, contrast, granularity, degree of after glow, and the like. Of these characteristics, a strong demand exists on the improvement of granularity since it greatly influences resolution and structure noise. However, it is difficult with the conventional method to manufacture an output screen having improved granularity. This is because it is impossible to reduce the average particle size of the phosphor, which is to be coated on face plate 11, to less than 1-3 .mu.m. In other words, the structure of the phosphor layer formed on plate 11 cannot be further improved. With the existing technology it is almost impossible to produce a phosphor screen which exhibits a better granuality or a higher resolution.
Japanese Patent Disclosure No. 47-38683 discloses a method of forming a thin phosphor film made of manganese activated zinc silicate. This method is designed to improve both the granuality and the resolution of an output screen for use in an X-ray image intensifier. In this method, powder of a mixture of zinc fluoride and manganese or manganese compound is vapor-deposited on a quartz glass substrate. The deposit is baked at 1000.degree. to 1200.degree. C. in an air atmosphere, thereby forming an output film made of Zn.sub.2 SiO.sub.4 :Mn phosphor.
The inventors hereof made output phosphor films by the method disclosed in Japanese Patent Disclosure No. 47-38683. A beam current was applied to these films at the density of 0.01 .mu.A/cm.sup.2, under accelerating voltage of 30 kV, and the cathode-luminescence brightness of each film was measured. The average brightness of these films was 10 to 20 cd/m.sup.2. This value is considered not sufficient to guarantee a practical use of the films in X-ray image intensifiers, for two reasons. First, an output efficiency of fluorescence in the thin phosphor film is low as compared with a film made of granular phosphor. Secondly, the luminous efficiency of the Zn.sub.2 SiO.sub.4 :Mn phosphor is only 6 to 8%. A film of this phosphor can hardly exhibit a practically adequate luminous efficiency if made thin.